Binary pulse trains used for the transmission of data or quantized voice samples, e.g. from a PCM terminal, are often transcoded for the purpose of avoiding the occurrence of long series of identical bits which frequently come into existence with natural coding. Thus, an extended all-zero or all-one sequence may result in loss of synchronism at the receiving station; other disadvantages include significant fluctuations in the average transmitting power and difficulties in detecting bit errors at the receiving end.
Particularly in the field of fiber-optical signaling, various codes have already been proposed for solving these problems. See, for example, an article entitled "Optical Pulse Formats for Fiber-Optical Digital Communications" by Yoshitaka Takasaki et al, published Apr. 4, 1976 in IEEE Transactions on Communications, Vol. Com. 24, p. 404.
The known codes, however, require a substantially higher optical power which may exceed by about 1.5 to 7.2 dB the power needed for the transmission of a pure binary code; see Table V of the aforementioned article. Moreover, these prior systems generally fail to satisfy the need for a quick and efficient modification of the decoding circuitry in response to detection of an excessive number of bit errors.